This invention relates generally to vehicle moisture detection systems which detect precipitation on an outer surface of a vehicle window or windshield and, more particularly, to a rain detector which is decoupled from the windshield and capable of detecting rain on an exterior surface of the window.
Several rain sensor systems have been proposed to date. Early systems were typically closely coupled to the interior surface of the windshield, such as by bonding to the glass surface or the like. By positioning the system immediately adjacent to the window, the rain to sensor units sampled a relatively small area on the window. However, even the small sampling area may include many scratches, pits or other surface irregularities on the window which may dominate the signal received by the rain sensor system, thereby leading to an erroneous detection of rain on the window when little or no rain is actually present. Furthermore, in order to achieve an adequate sampling area, a greater number of sampling channels is required, which results in an increase of cost and bulk to the units. An additional concern with closely coupled units is that they create difficulties in the vehicle assembly plants, since it is then necessary to handle two different windshield configurations in the factory, which increases inventory costs. The close coupling also creates difficulties in the replacement of the windshield in the after market, since the rain sensor unit has to be replaced along with the windshield if the windshield is damaged.
Other rain sensor devices have been proposed that are decoupled from the windshield such that the sensors are spaced from the interior surface of the windshield, in order to avoid concerns with replacing the windshield and other deficiencies present with the coupled systems. However, by spacing the rain sensor from the interior of the surface of the window, the rain sensor receives data from a larger sampling area on the window. This further increases the likelihood of significant errors in detecting rain droplets on the exterior of the windshield and discerning them from scratches or other surface irregularities which may be present on the window. This is a greater concern when the rain sensor is operable on a windshield of a vehicle, since the exterior surface of a windshield is typically scratched and/or nicked in multiple places due to debris impacting the windshield as the vehicle is driven. Because the number of surface irregularities may be significantly greater than the amount of precipitation that may be present on the window, the rain sensor systems may result in erroneous detection of rain droplets when there is little or no precipitation present on the window.
Other systems have recently been proposed which include an illumination source and an illumination sensor at an acute angle relative to one another, such that when the light from the illumination source is refracted through the windshield and further reflected by water droplets on the exterior surface of the windshield, the light may be received by the illumination sensor and processed to determine if precipitation is present on the exterior surface of the windshield. However, the addition of an illumination source further intensifies the appearance of the surface irregularities which may be present on the window, since the light reflects and scatters from the pits or scratches back toward the sensor, such that there is a greater likelihood that the surface irregularities will dominate the signal received by the illumination sensor. This again may result in an erroneous detection of precipitation when there is little or no rain present on the window, since none of these systems account for any surface irregularities, such as scratches or pits or the like, on the exterior and/or interior surfaces of the window.